SELECT(2) BSD System Calls Manual SELECT(2)
NAMEFD_CLR, FD_COPY, FD_ISSET, FD_SET, FD_ZERO, select -- synchronous I/O multiplexing
SYNOPSIS#include<sys/select.h>voidFD_CLR(fd, fd_set*fdset);
voidFD_COPY(fd_set*fdset_orig, fd_set*fdset_copy);
intFD_ISSET(fd, fd_set*fdset);
voidFD_SET(fd, fd_set*fdset);
voidFD_ZERO(fd_set*fdset);
intselect(intnfds, fd_set*restrictreadfds, fd_set*restrictwritefds, fd_set*restricterrorfds,
structtimeval*restricttimeout);
DESCRIPTIONSelect() examines the I/O descriptor sets whose addresses are passed in readfds, writefds, and errorfds
to see if some of their descriptors are ready for reading, are ready for writing, or have an excep-tional exceptional
tional condition pending, respectively. The first nfds descriptors are checked in each set; i.e., the
descriptors from 0 through nfds-1 in the descriptor sets are examined. (Example: If you have set two
file descriptors "4" and "17", nfds should not be "2", but rather "17 + 1" or "18".) On return,
select() replaces the given descriptor sets with subsets consisting of those descriptors that are ready
for the requested operation. Select() returns the total number of ready descriptors in all the sets.
The descriptor sets are stored as bit fields in arrays of integers. The following macros are provided
for manipulating such descriptor sets: FD_ZERO(&fdset) initializes a descriptor set fdset to the null
set. FD_SET(fd, &fdset) includes a particular descriptor fd in fdset. FD_CLR(fd, &fdset) removes fd
from fdset. FD_ISSET(fd, &fdset) is non-zero if fd is a member of fdset, zero otherwise.
FD_COPY(&fdset_orig, &fdset_copy) replaces an already allocated &fdset_copy file descriptor set with a
copy of &fdset_orig. The behavior of these macros is undefined if a descriptor value is less than zero
or greater than or equal to FD_SETSIZE, which is normally at least equal to the maximum number of
descriptors supported by the system.
If timeout is a non-nil pointer, it specifies a maximum interval to wait for the selection to complete.
If timeout is a nil pointer, the select blocks indefinitely. To effect a poll, the timeout argument
should be non-nil, pointing to a zero-valued timeval structure. Timeout is not changed by select(),
and may be reused on subsequent calls, however it is good style to re-initialize it before each invoca-tion invocation
tion of select().
Any of readfds, writefds, and errorfds may be given as nil pointers if no descriptors are of interest.
RETURNVALUESSelect() returns the number of ready descriptors that are contained in the descriptor sets, or -1 if an
error occurred. If the time limit expires, select() returns 0. If select() returns with an error,
including one due to an interrupted call, the descriptor sets will be unmodified and the global vari-able variable
able errno will be set to indicate the error.
ERRORS
An error return from select() indicates:
[EAGAIN] The kernel was (perhaps temporarily) unable to allocate the requested number of file
descriptors.
[EBADF] One of the descriptor sets specified an invalid descriptor.
[EINTR] A signal was delivered before the time limit expired and before any of the selected
events occurred.
[EINVAL] The specified time limit is invalid. One of its components is negative or too
large.
[EINVAL] ndfs is greater than FD_SETSIZE and _DARWIN_UNLIMITED_SELECT is not defined.
LEGACYSYNOPSIS#include<sys/select.h>
- or -#include or#include#include<sys/types.h>#include<sys/time.h>#include<unistd.h>FD_SET(fd, &fdset);
FD_CLR(fd, &fdset);
FD_ISSET(fd, &fdset);
FD_COPY(&fdset_orig, &fdset_copy);
FD_ZERO(&fdset);
COMPATIBILITYselect() now returns with errno set to EINVAL when nfds is greater than FD_SETSIZE. Use a smaller
value for nfds or compile with -D_DARWIN_UNLIMITED_SELECT.
SEEALSOaccept(2), connect(2), getdtablesize(2), gettimeofday(2), read(2), recv(2), send(2), write(2),
compat(5)BUGS
Although the provision of getdtablesize(2) was intended to allow user programs to be written indepen-dent independent
dent of the kernel limit on the number of open files, the dimension of a sufficiently large bit field
for select remains a problem. The default size FD_SETSIZE (currently 1024) is somewhat smaller than
the current kernel limit to the number of open files. However, in order to accommodate programs which
might potentially use a larger number of open files with select, it is possible to increase this size
within a program by providing a larger definition of FD_SETSIZE before the inclusion of <sys/types.h>.
Select() should probably have been designed to return the time remaining from the original timeout, if
any, by modifying the time value in place. However, it is unlikely this semantic will ever be imple-mented, implemented,
mented, as the change would cause source code compatibility problems. In general it is unwise to
assume that the timeout value will be unmodified by the select() call, and the caller should reinitial-ize reinitialize
ize it on each invocation.
HISTORY
The select() function call appeared in 4.2BSD.
4.2 Berkeley Distribution March 25, 1994 4.2 Berkeley Distribution

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